[Paper Review] Peculiar Red Nova V838 Mon: Accretion and Interaction in a Wide Binary System after Explosion of Its Companion

V. P. Goranskij, Α. V. Zharova+3 more|ArXiv.org|Oct 10, 2008

Astrophysical Phenomena and Observations1 references3 citations

TL;DR

The paper proposes that the 2006 eclipse-like event in V838 Mon was caused by a transient, highly inclined accretion disc forming around the B3V companion star due to infalling ejecta from the 2002 outburst. Subsequent disappearance of the companion from 2007 onward indicates its engulfment by the expanding cool remnant, with ongoing heating of a new thick accretion disc, evidenced by persistent [FeII] emission and increasing red excess in photometry.

ABSTRACT

We report the results of recent multicolor photometry and medium resolution spectroscopy of V838 Mon taken in 2007-2008. In the eclipse-like event in December 2006, the hot B3V type companion disappeared. The event accompanied by strengthening emission [FeII]/FeII lines in the spectra. We explain this event as the formation of temporal short-lived accretion disc around the hot companion. Later, in February 2007 the hot star reappeared in its full brightness, but disappeared again for a long time since September 2007. This is the engulf of B3V companion by expanding remnant of 2002 outburst. We assume that the thick accretion disc has formed around B type companion which is moving now inside the envelope of the cool star. There is some evidence of heating this disc and/or cool star envelope. We estimated the radius of expanding cool remnant in December, 2006 of about 150 A.U. or 30000 solar radii.

Motivation & Objective

• To explain the 2006 eclipse-like event in V838 Mon, where the B3V companion temporarily vanished from optical spectra.
• To investigate the nature of the strong [FeII]/FeII emission lines observed during the 2006 event and their implications for accretion processes.
• To determine whether the B3V companion was temporarily obscured by a dust cloud, a transient disc, or engulfed by the expanding cool remnant.
• To assess the long-term evolution of the system post-2002 outburst, particularly the interaction between the hot companion and the cool stellar envelope.
• To model the photometric and spectroscopic evolution of V838 Mon from 2004 to 2008, focusing on the formation and dissipation of accretion structures.

Proposed method

• Multicolor photometry in UBVR_C I_C bands was conducted using the 1-m Zeiss reflector at SAO and 60-cm Zeiss reflector to track brightness changes from 2004 to 2008.
• Medium-resolution spectroscopy (13 Å resolution) was obtained with the 6-m BTA telescope to analyze emission and absorption features in the optical spectrum.
• Spectral energy distributions (SEDs) were constructed to compare the pre-outburst binary system, the post-outburst remnant, and the evolving system during the 2006–2008 period.
• The expansion velocity of 145 km/s from high-resolution IR CO band measurements (Geballe et al. 2007) was used to estimate the radius of the cool remnant at 30,000 R⊙ in December 2006.
• Orbital parameters were estimated assuming a circular orbit with a separation of 150 AU, yielding an orbital period of ~500 years and velocity of ~10 km/s.
• The presence of forbidden FeII lines and Balmer emission was interpreted as evidence of ionized gas in a transient or persistent accretion disc around the B3V star.

Experimental results

Research questions

• RQ1 What caused the 2006 eclipse-like event in V838 Mon, where the B3V companion disappeared from the optical spectrum?
• RQ2 What is the origin of the strong [FeII]/FeII emission lines observed during the 2006 event, and what do they reveal about the accretion process?
• RQ3 Why did the B3V companion reappear in February 2007 but then disappear again from September 2007 onward?
• RQ4 How does the interaction between the B3V companion and the expanding cool remnant evolve over time, particularly in terms of accretion disc formation and heating?
• RQ5 What is the nature of the increasing red excess in photometry from late 2006 to 2008, and how does it relate to the evolution of the accretion disc?

Key findings

• The 2006 eclipse-like event, with a 1.7 mag decline in the U band and 70-day duration, was caused by a transient, highly inclined accretion disc obscuring the B3V companion from Earth’s line of sight.
• The B3V companion reappeared in February 2007, indicating the disc had dissipated, but re-disappeared by September 2007, suggesting it was engulfed by the expanding cool remnant.
• The radius of the cool remnant was estimated at 30,000 R⊙ in December 2006, based on the 145 km/s expansion velocity from CO band measurements.
• Spectroscopic observations in February 2007 showed strong [FeII], FeII, and Hβ emission, while by February 2008, these lines were 10 times fainter, indicating a fading accretion disc.
• The 2008 spectrum revealed a weak blue continuum and broad Balmer absorptions, consistent with a B3V star seen through a cloudy, ionized shell with an inner void.
• The system exhibited a growing red excess from late 2006 onward, attributed to heating of a new, thick accretion disc forming around the B3V star as it moved within the cool remnant’s envelope.